Measuring systems which measure various physical values are employed for determining the fill level of a product in a container. The desired information regarding the fill level is subsequently derived from these values. Capacitive, conductive or hydrostatic measuring probes are employed, besides mechanical detectors, as well as detectors operating on the basis of ultrasound, microwaves or radioactive radiation.
Highly accurate measurements of the fill level of fluids or bulk materials in containers (tanks, silos, etc.) are demanded in a multitude of fields, for example in petroleum chemistry, chemistry and food industry. For this reason sensors are employed in increasing numbers there, in connection with which short electromagnetic high-frequency pulses (TDR method or pulse-radar method) or continuous frequency-modulated microwaves (for example FMCW-radar method) are excited in a conductive element, or a waveguide, and are introduced by means of the waveguide into the container in which the product is stored. The known variants of surface waveguides of Sommerfeld or Goubau or Lecher waveguides are possible waveguides.
From a physical point of view, this measuring method makes use of the effect that a portion of the guided high-frequency pulses, or of the guided microwaves, is reflected at the interface between two different media, for example air and oil, or air and water, because of the sudden change (discontinuity) of the dielectric numbers of both media, and is guided back into the receiving unit via the conductive element. The reflected portion (→effective echo signal) is all the greater, the greater the difference in the dielectric numbers of the two media is. The distance to the surface of the product can be determined by means of the running time of the reflected portion of the high-frequency pulses, or the CW signals (echo signals). Knowing the empty distance of the container, it is possible to calculate the fill level of the product in the container. If an interface determination is to be produced, the position of the interface can be determined by means of the measured results.
Sensors with guided high-frequency signals (the signals can be pulses or waves) are distinguished over sensors which freely radiate high-frequency pulses or waves (free-field microwave systems (FMR), or “real radar systems”) by a considerably higher echo amplitude. The reason for this is that the output flow takes place quite specifically along the waveguide, or the conductive element. Moreover, in connection with bulk materials with inclined surfaces (conical pile, draw-off funnel) or liquids with moving surfaces (bubbles, eddies, waves), the danger of retro-reflections is ruled out when using guided high-frequency signals.
For performing the measurement, the conductive element is customarily in direct contact with the medium. If it is a medium which can be statically charged, for example bulk material of low electrical conductivity, for example plastic granules or cement (by the way, the static charge is particularly pronounced in case of pneumatic filling), it can occur that static discharges in the measured medium are drawn off via the conductive element in the direction of the signal generating unit, or the receiving/evaluating unit. In that case the danger exists that the electronic and electric parts of the respective units, which are particularly sensitive to voltage peaks, are destroyed. It can occur in the other direction that in case of a defect of the electronic part the full operating voltage of, for example 220 V, is present at the conductive element, which results in an enormous danger for the operators.